The field of the invention is an integrated circuit with a test interface and a method of testing power supply connections in a circuit that contains such an integrated circuit.
To ensure that an integrated circuit will work properly under all conditions, it is desirable to test the IC for faults in the power supply connections. In principle a completely malfunctioning power supply connection can be detected by a lack of response from functional blocks that should receive power from the power supply connection. However, such a test does not cover all possible faults. For example, if the integrated circuit has more than one power supply connection for the same supply voltage, a fault in one connection may be masked because supply current could flow to the functional block from another supply connection. This might allow the functional block to respond under some circumstances, although it is unable to operate adequately in certain situations, for example when it suddenly has to consume an increased supply current.
As an alternative, one may monitor the voltage drop along the power supply connection. But such a test for faulty power supply connections in an integrated circuit is difficult, because no voltage drop other than parasitic voltage drops can be tolerated along power supply connections. U.S. Pat. Nos. 5,894,224 and 5,963,038 describe a technique, which uses coils to detect the current along the power supply connection, but this only works in integrated circuit technologies that allow for incorporation of coils without much parasitic effects.
U.S. Pat. No. 5,068,604 discloses a method of testing the functioning of power supply connections of an integrated circuit that has multiple supply connections for the same power supply voltage. The test uses a measurement of the internal voltage difference in the integrated circuit between the voltage of different nodes that should be connected to different power supply connections for the same supply voltage. The idea is that a current will flow in the integrated circuit between these nodes in case one of the power supply connections is not connected properly. In the connections between such nodes a higher resistance can be tolerated, which results in a measurable voltage drop between the nodes.
Unfortunately, it is not always straightforward to interpret such a voltage drop. For example, if there are three or more connections for the same power supply voltage connected to respective nodes, current might flow between a first and second one of the nodes both when the second node is disconnected from its power supply connection and when a third one of the nodes is disconnected. To resolve the fault, an additional measurement may be needed.
Amongst others, it is an object of the invention to provide for an integrated circuit and a method of testing power supply connections to integrated circuits wherein a more straightforward test is possible.
The integrated circuit according to the invention has a current test circuit with a threshold shifting circuit for shifting the threshold of the current test circuit into an operating range before comparing a voltage across the test input of the current test circuit with the threshold. The threshold shifting circuit is triggered as integral part of the response to a test command, which is preferably received via a conventional scan chain test interface (as used herein, xe2x80x9cscan testxe2x80x9d encompasses both internal boundary scan test (aimed at circuit board level testing) and scan test of circuits internal to an integrated circuit). Thus, the threshold is dynamically adjusted as part of execution of the test command. This makes it possible to use highly sensitive current detection.
In an embodiment the integrated circuit according to the invention performs said shifting by applying a predetermined input voltage to the current test circuit and shifting the threshold so that the current test circuit has its threshold set at a level that is at a predetermined difference from the input voltage. In a preferred embodiment the predetermined input voltage is zero. In this way, a desired threshold can be easily controlled. Preferably, the threshold is adjusted in several steps, first bringing it to a level which equals the input voltage when a predetermined reference voltage is present at the test input and then shifting the threshold by a predetermined amount.
Such a current test circuit may be used to test the amount of xe2x80x9csurgexe2x80x9d current caused by switching on output buffers in the functional block. Output buffers are connected to terminals that constitute relatively large capacitive loads, such as external output pins of the integrated circuit. Charging such a load causes a temporary surge current, which may lead to power supply bounce effects. By using the current along the power supply connection either to shift the threshold or for comparison with the threshold a predetermined time interval after switching on the output buffers, it is possible to judge whether this current may lead to ground bounce effects. Preferably, the integrated circuit contains a timer to control the time between switching on current through the output buffers and its use.
Of course the current test circuit may also be used to perform other tests, such as a test whether the power supply connection actually conducts current. Preferably, to accommodate different kinds of test, the threshold shifting circuit is arranged to offset the threshold of the current test circuits by different selectable predetermined amounts from a voltage across the inputs of the current test circuit when the threshold is shifted.
In another embodiment of the circuit according to the invention a shunt circuit is arranged in parallel with the functional circuit to ensure sufficient current for detection. The shunt circuit is preferably a current source that is switched on to ensure a precise current. The shunt circuit allows for an accurately controlled increase in the supply current through the connection under test during testing, to ensure a sufficiently large voltage drop. The words xe2x80x9cshunt circuitxe2x80x9d are understood to mean any circuit arranged to draw current in parallel with another circuit. The words shunt circuit should not be understood to be limited to a short circuit, which causes an appreciable voltage drop over the circuit that is shunted by the shunt circuit.
In a further embodiment, the circuit has several supply pads and several shunt circuits, each for specific one of the supply pads. The supply pads are connected in the integrated circuit by a power supply conductor, to which the shunt circuits are also connected. When a test command to test current from one of the supply pads is executed, all shunt circuits may be activated to ensure that current from other supply pads doesn""t disturb the test. However, this leads to considerable power dissipation in the integrated circuit. In an embodiment the only activated shunt circuit(s) are the shunt circuits for one or more neighboring power supply pads, that are connected to the power supply conductor nearest to the supply pad under test. Shunt circuits xe2x80x9cfurther onxe2x80x9d are kept non-conductive. In an embodiment, current detection circuits for the supply pads corresponding to the other activated shunt circuits are self tested while they are activated for this test.